1. Field of the Invention
The present invention relates to a directional coupler which is applied to a dual-band mobile communication terminal such as a dual-band mobile phone, and more particularly to a directional coupler which is implemented with strip lines for signal coupling and inter-digital capacitors for phase compensation so that it can be improved in directivity, minimized in process error and miniaturized to be readily implemented in one-chip form, and a dual-band transmitter using the same.
2. Description of the Related Art
In general, a power amplifier is used in a transmitter of a mobile communication terminal, such as a mobile phone, to amplify power of a transmit signal to be sent out through an antenna of the terminal. This power amplifier has to amplify the transmit signal to an appropriate power level. Methods for regulating output power of the power amplifier can be roughly classified into two types, a closed loop type of detecting a part of an output signal from an output port of the power amplifier through a directional coupler, converting the detected signal into direct current (DC) current using a Schottky diode and comparing the converted DC current with a reference voltage through a comparator, and an open loop type of regulating power by sensing a voltage or current applied to the power amplifier.
The closed loop method is a traditional method and has the advantage of being able to finely control power, but the disadvantage of involving complexity in circuit implementation and degrading efficiency of the amplifier due to a loss by the coupler. The open loop method is currently often used in that it involves simplicity in circuit implementation, but has the disadvantage of being unable to finely control power.
Recently, components used in the closed loop method have been provided in integrated circuit (IC) form, thereby making circuit implementation simple. Further, the performance of a control chip has become better, thereby significantly lowering the coupling value of the directional coupler, resulting in a significant reduction in loss by the coupler. Particularly, the closed loop method capable of finely controlling power has been applied to a GSM (Global System for Mobile) communication system where much attention is given to a ramping profile.
A transmitter with a power control function of the above-mentioned closed loop type will hereinafter be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the configuration of a conventional transmitter.
As shown in FIG. 1, the conventional transmitter comprises a power amplifier 11 for amplifying power of a transmit signal ST, a directional coupler 12 for coupling a part of an output signal from the power amplifier 11, a power controller 13 for controlling an amplification factor of the power amplifier 11 on the basis of the level of a signal coupled by the directional coupler 12, and a filter 14 for receiving the output signal from the power amplifier 11 through the directional coupler 12 and passing it to an antenna ANT.
Recently, a dual-band terminal has been developed which is capable of transmitting and receiving both signals of two bands, for example, a high band, such as a frequency band of a DCS (Digital Cellular System) 1800 communication system using about 1800 MHz, and a low band, such as a frequency band of a GSM communication system using about 900 MHz.
This dual-band terminal requires a directional coupler which is capable of coupling a signal of each of the two bands to control power of each band. Such a directional coupler for the dual-band terminal must have good directivity and inter-band isolation characteristics.
One such directional coupler which is applied to the dual-band terminal will hereinafter be described with reference to FIG. 2.
FIG. 2 is a layout view of a conventional directional coupler.
The conventional directional coupler shown in FIG. 2, denoted by the reference numeral 20, is adapted to couple a part of a signal between a first input port 1 and a first output port 2 and a part of a signal between a second input port 4 and a second output port 5, respectively, and output the coupled signals through a coupling port 3. To this end, the directional coupler 20 includes a first band signal line SL1, a second band signal line SL2, and a coupling line SL3 disposed between the two band signal lines SL1 and SL2 adjacently thereto. The coupling line SL3 is used in common for two bands, and has its one port connected to the coupling port 3 and its other port connected to a ground terminal via a resistor RT of 50Ω. This directional coupler has a coupling factor which is determined depending on the distance between the coupling line and each signal line and the length of the coupling line, which is typically λ/4.
A detailed description of this directional coupler is shown in European Patent No. 0,859,464 A3.
The directional coupler 20, which is typically applied to a dual-band transmitter, outputs coupled signals of two bands through one coupling port by using one coupling line. As a result, the coupler itself is reduced in size and a power controller including a detecting diode, comparator, etc. is simplified in construction, too. That is, this coupling structure is more concise and simpler in terms of size than a structure for individual coupling by bands.
However, since the coupling port is used in common for the two bands in the conventional directional coupler for the dual-band transmitter to reduce the chip size of the coupler, there is a problem in that an inter-band isolation is reduced in the dual-band transmitter.
A filter-type directional coupler using a diplexer, as shown in FIG. 3, has been proposed to improve the inter-band isolation in the dual-band transmitter.
FIG. 3 is a schematic view of a conventional filter-type directional coupler.
The conventional filter-type directional coupler shown in FIG. 3, denoted by the reference numeral 30, includes a first coupling capacitor C1 for coupling a part of a signal between a first input port 1 and a first output port 2, a second coupling capacitor C2 for coupling a part of a signal between a second input port 4 and a second output port 5, and a diplexer 31 for outputting signals coupled by the first and second coupling capacitors C1 and C2 through a coupling port 3. The diplexer 31 includes a first filter FT1 for high pass filtering the signal coupled by the first coupling capacitor C1, and a second filter FT2 for low pass filtering the signal coupled by the second coupling capacitor C2.
In this conventional filter-type directional coupler, each of the filters selectively passes only a corresponding one of the two bands and blocks the other band, thereby making the isolation between the two bands good.
In general, a directional coupler for a mobile communication terminal such as a mobile phone couples a very small amount of power necessary for power control, for example, about −33 dB or −28 dB, which leads to a coupling loss of about −0.02 dB. Considering a loss on a transmission line, a reflection loss due to mismatch, etc., a small coupling loss of about −0.05 to −0.1 dB appears.
However, the above-mentioned conventional filter-type directional coupler is disadvantageous in that it is increased in chip size and degraded in directivity, as will hereinafter be described with reference to FIGS. 4a to 4d. 
Shown in FIGS. 4a to 4d are characteristics of the filter-type directional coupler of FIG. 3 in the case where a DCS band signal is transmitted through the first input port 1 and first output port 2, a GSM band signal is transmitted through the second input port 4 and second output port 5 and coupled signals of the DCS band signal and GSM band signal are outputted through the coupling port 3.
FIGS. 4a to 4d are views showing characteristics of the filter-type directional coupler of FIG. 3.
In FIG. 4a, S(2,1) and S(5,4) are insertion losses of DCS and GSM bands, respectively. In FIG. 4b, S(3,1) is a coupled value of DCS 1800 MHz, and S(3,2) is an extracted power value appearing at the DCS band output port. Here, the difference between S(3,1) and S(3,2) signifies directivity. In FIG. 4c, S(1,4) is an inter-band isolation. In FIG. 4d, S(3,4) is a coupled value of the GSM band, and S(3,5) is an extracted power value appearing at the GSM band output port. Here, the difference between S(3,4) and S(3,5) signifies directivity. S(P1,P2), where P1 and P2 mean ports, signifies the amount of a signal of the port P2 which is partially sent to the port P1. For example, S(3,1) represents the amount of a signal which is sent from the port 1 to the port 3.
In the conventional filter-type directional coupler, however, in order to extract a low coupled value of about −33 dB, it is necessary to shorten a strip line and space a signal line and a coupling line away from each other. In this case, the directivities, which are the difference between S(3,2) and S(3,1) and the difference between S(3,4) and S(3,5), appear as low values of about 0 to −1 dB, as shown in FIGS. 4b and 4d. As a result, the conventional filter-type directional coupler has the disadvantage of not being good in directivity and the disadvantage of being increased in chip size.